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Equity Compensation Plans - Restricted Stock (Details Textual) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Weighted-Average Grant Date Fair Value, Awards granted | $ 40.86 | $ 41.69 | $ 45.57
Market value of restricted stock shares vesting | $ 32.8 | $ 17.0 | $ 22.7
Restricted stock |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Weighted-Average Grant Date Fair Value, Awards granted | $ 40.86 |  |
Market-based Awards |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Vesting period | 4 years |  |
Performance based awards granted to executive management and certain key employees shares | 0.1 |  |
Performance Shares |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Vesting period | 2 years |  |
Minimum | Restricted stock |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Vesting period | 2 years |  |
Maximum | Restricted stock |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Vesting period | 4 years |  |

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- Definition Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Net number of share options (or share units) granted during the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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